This invention relates to a process for producing a 2-pentenoic ester which comprises isomerizing the corresponding 3-pentenoic ester.
In general, many prior processes for isomerizing unsaturated carboxylates are known. For example, (a) J. Chem. Soc. 2454 (1957) discloses a process for isomerizing a 3-hexenoic ester in the presence of potassium hydroxide as a catalyst; (b) J. Org. Chem. 35 3352 (1970) discloses a process for isomerizing methyl 3-pentenoic ester in the presence of sodium methylate or iron pentacarbonyl; and (c) Bull. Chem. Soc. Jpn., 51 2970 (1978) discloses a process for isomerizing dimethyl methylenesuccinate to dimethyl 2-methylbutenoate by using triethylamine.
However, in process (a), the ester employed as a starting material is hydrolyzed by the alkali hydroxide to form the alkali salt of unsaturated carboxylic acid. In order to avoid this shortcoming, the unsaturated carboxylate which is a starting material should be neutrilized, isomerized and esterified. This is complicated.
In process (b) in which sodium methylate is used, sodium methylate adds to the double bond of the unsaturated carboxylate. This lowers the yield of 2-pentenoic ester. In process (b) in which iron pentacarbonyl is used, the reaction speed is slow and, also, the reaction is necessarily effected under an atmosphere pressurized by carbon monoxide, because iron pentacarbonyl is unstable.
In process (c), isomerization of dimethyl methylenesuccinate to dimethyl 2-methylbutenoate proceeds in the presence of triethylamine even at room temperature, and little side reaction occurs. However, isomerization reaction of a 3-pentenoic ester to the corresponding 2-pentenoic ester in the presence of triethylamine alowly proceeds at 100.degree. C. This is too slow to be industrially practicable.